It is sought, in the context of the design of personalized ophthalmic vision-correcting lenses, to take into account individual geometrico-postural parameters of the wearer (referred to as personalized optical design parameters) and the frame he has chosen.
The sought-after geometrico-postural parameters are related not only to the geometry of the head of the wearer and to that of the chosen spectacle frame, but also to the posture of the wearer and to his visual behavior.
In order to determine these parameters, the values of the following quantities are in particular determined: height of the eyes of the wearer with respect to the lower edge of the lens or of the spectacle frame, interpupillary distance, position of the eye rotation centers, eye-head coefficient (i.e. the ratio between the angle of rotation of the head and the angle of rotation of the eyes during the reading of a text, or more generally, during a visual stimulus that is off center with respect to a reference visual axis corresponding to a straight-ahead gaze direction), value of the “inset” for a progressive ophthalmic lens, distance between the back face of the ophthalmic lens and the top of the cornea of the eye.
“Inset” is defined in the standard ISO13666: 2012 as the horizontal distance between the fitting cross and the center of the designed near zone. “Inset” is also called “internal offset”. It depends on the visual behavior of the wearer.
Ophthalmic progressive lenses allow the wearer to benefit from an optical power correction that is adapted to various vision distances without changing spectacles. They may also correct other visual defects, such as astigmatism for example.
A progressive ophthalmic lens has a variable power over the surface of the lens.
For example provision is made for a first vision zone for far vision having a first average power value, a second vision zone for near vision having a second average power value and, between these two zones, a third vision zone for intermediate vision, the curvature of which varies gradually and which is called the progression corridor.
The fitting height of the ophthalmic lens corresponds to the height, relative to the lower edge of the rim of the frame, of the projection of the pupil of the wearer having a predetermined primary gaze direction onto a mean plane of this rim of the chosen frame, corresponding to a mean surface or mean plane of the ophthalmic lens once fitted into said frame.
This predetermined primary gaze direction corresponds to the gaze direction of the wearer under far-vision conditions.
Thus, the positions of the near- and far-vision zones of the wearer are parameters that are very important for the personalized design of progressive ophthalmic lenses. These near- and far-vision zones of the wearer constitute two zones of use of the ophthalmic lens.
Devices for tracking wearer gaze direction belonging to measuring-totem or measuring-tablet type families of optical measuring devices are known.
However, these devices for tracking gaze direction have the drawback of not allowing this gaze direction to be tracked under natural, i.e. unconstrained, wearer visual behavior and posture conditions since the wearer must stand up or sit down in front of the measuring totem or hold the measuring tablet in his hands.